Coil coated metal is often used as a building material in commercial buildings, and in the private home sector. Coil coated metal is very durable and requires very little maintenance, however, many of the buildings constructed with coil coated metal are beginning to show signs of weathering due to extended service. The weathered coatings of such buildings are fading and losing their capacity to protect the metal. Because the protective coatings used as the topcoat in coil coated metals are designed to have long service lives, they form relatively stable and chemically inert surfaces. These surfaces are difficult to recoat. Existing paint primers are not completely satisfactory in solving the problem of repainting coil coated surfaces. Existing paint primers allow one to recoat with a paint less durable and weatherable than the original coil coating, but they do not provide sufficient adhesion when one uses high performance paint. In order to extend the service life of buildings made from coil coated metal a method of effectively repainting such surfaces is required.
Coil coated metal is made by unrolling a coil of metal and treating the surface with a conversion coating such as zinc phosphate. The conversion coating is baked onto the metal surface, and a primer coat is applied and baked onto the surface (this primer coat should not be confused with the primer coat of the invention which is applied over the protective coating, rather than under it). Finally, a protective topcoat is applied and baked onto the surface and the metal is recoiled for shipping.
The coil coated surfaces of buildings weather in an uneven fashion. The area under the eaves of the building will not be as weathered as the portions of the building with the greatest southern exposure. Therefore, it is necessary to provide a primer that will allow new paint to adhere to both weathered and unweathered coil coated surfaces.
The present invention provides a method for painting coil coated surfaces with paints which are as durable and easy to maintain as the original coil coatings. The method of painting disclosed in the present application can be practiced in place on coil coated metal which has already been incorporated into buildings, thus allowing such surfaces to be repainted in the same manner as conventional buildings are repainted. Alternately, the method of the present application can be practiced to recoat coil coated metal which has not been installed.